Conventionally, in the step for producing a secondary battery such as a lithium ion secondary battery, a wound body made by winding a positive electrode, a negative electrode and a separator is laterally housed in a battery case.
In the step for producing the secondary battery, the battery case is sealed after pouring the electrolyte solution into the battery case, and the wound body is impregnated with the electrolyte solution. Further, in the step for producing the secondary battery, initial charging of the secondary battery is carried out. At this time, a membrane is formed on the wound body by decomposition reaction of the electrolyte solution.
In a technique disclosed in Patent Literature 1, an opening of a case (battery case) is closed to reduce the internal pressure of the case, and then an electrolyte solution is poured into the case whose internal pressure is reduced.
In the technique disclosed in Patent Literature 1, the internal pressure of the case is increased to a pressure higher than an atmospheric pressure while pouring the electrolyte solution (or simultaneously with the pour of the electrolyte solution), and thereby the electrolyte solution is impregnated into the wound body.
Immediately after pouring the electrolyte solution, the electrolyte solution permeates into both axial end portions of the wound body by a capillary phenomenon. Thereby, a sealed space is formed inside the wound body.
In the case where the internal pressure of the case is increased while pouring the electrolyte solution as in the technique disclosed in Patent Literature 1, the pressurized air inside the case may push the electrolyte solution that has permeated into both the axial end portions of the wound body away, thereby raising a possibility that the air may penetrate into the wound body.
In other words, in this case, there is a possibility that the electrolyte solution cannot be impregnated into the path of air penetration and the axial middle portion of the wound body.
For this reason, by the technique disclosed in Patent Literature 1, a uniform membrane may not be formed on the wound body.